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Concentrator photovoltaics

Concentrator systems use mirrors or lenses to bundle the solar radiation and focus it on special concentrator solar cells. Compared with conventional cells, the necessary semiconductor surface area is reduced to around five-hundredth of the size. Researchers from Fraunhofer ISE have achieved a record efficiency of 41.1% with a metamorphic triple-junction solar cell made from the III-V compound semiconductors gallium indium phosphide, gallium indium arsenide and germanium.

Cell stack and concentrator

To achieve this high efficiency, several solar cells are stacked on top of one another. The triple-junction solar cell consists of more than 20 individual layers. In concentrator modules, a Fresnel lens situated roughly ten centimetres above the 3mm² small solar cells concentrates the sunlight by between 400 and 500 times. The cells are mounted on a copper plate that dissipates the heat. Such modules achieve an efficiency of 29%.

For the efficiency of the new, very highly efficient structures, it is essential that the incident sunlight is divided into three equally large spectral ranges by using a suitable choice of absorbing materials. All sub-cells then generate the same amount of electricity. This is very important with serially connected solar cells, since the electricity produced by the component is always limited to the smallest amount of electricity produced by a single sub-cell. With the metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge materials, it has been possible to create a solar cell structure that, in terms of generating electricity, is completely matched under the terrestrial solar spectrum.

High-yield system for sunny regions

The new modules are particularly suitable for sunny regions with lots of direct solar radiation, e.g. in southern Europe. Mounted on a tracker, they track the sun on two axes so that the focal point of the lens always hits the active part of the solar cells.

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